Spread spectrum (SS) systems, which may be CDMA systems, are well known in the art. SS systems can employ a transmission technique in which a pseudo-noise (PN) code is used as a modulating waveform to spread the signal energy over a bandwidth much greater than the signal information bandwidth. At the receiver the signal is de-spread using a synchronized replica of the PN-code.
In general, there are two basic types of SS systems: direct sequence spread spectrum systems (DSSS) and frequency hop spread spectrum systems (FHSS).
The DSSS systems spread the signal over a bandwidth fRF±Rc, where fRF represents the carrier frequency and Rc represents the PN-code chip rate, which in turn may be an integer multiple of the symbol rate Rs. Multiple access systems employ DSSS techniques when transmitting multiple channels over the same frequency bandwidth to multiple receivers, each receiver sharing a common PN code or having its own designated PN-code. Although each receiver receives the entire frequency bandwidth, only the signal with the receiver's matching PN-code will appear intelligible; the rest appears as noise that is easily filtered. These systems are well known in the art and will not be discussed further.
FHSS systems employ a PN-code sequence generated at the modulator that is used in conjunction with an m-ary frequency shift keying (FSK) modulation to shift the carrier frequency fRF at a hopping rate Rh. A FHSS system divides the available bandwidth into N channels and hops between these channels according to the PN-code sequence. At each frequency hop time a PN generator feeds a frequency synthesizer a sequence of n chips that dictates one of 2n frequency positions. The receiver follows the same frequency hop pattern. FHSS systems are also well known in the art and need not be discussed further.
Generally, the data rate of the SS transmitted signal is known a priori. This is accomplished either by providing a fixed data rate system, or by varying the data rate in a predictable manner, such as by varying the data rate according to a particular time or day. The DSSS signal is then acquired and data extracted using a filter matched to the known data rate.
For applications when the current data rate is unknown, a sequential search of the data may be accomplished to determine if received demultiplexed data is valid. If the data is found to be invalid, then this can indicate that an incorrect data rate was assumed, resulting in another data rate being selected and tested.
Other known techniques involve sequentially searching PN codes until the correct code correlation is achieved. It will be appreciated that these PN codes are preferably relatively short, and not codes that repeat at time scales on the order of days or weeks. A “short” PN code, as defined herein by example only, is a PN code of length on the order of about 1000 chips that has a duration of about a second or less.
In one type of system of interest to the teachings of this invention a broadcast data link originates from a central airborne terminal (hereafter referred to as the Central Terminal) whose exact position is not necessarily known a priori by the intended receiver (hereafter referred to as the Receive Terminal).
A Receive Terminal, using a directional antenna, decreases beamwidth (i.e., search sector) with increased antenna gain to locate the Central Terminal and to receive the signal within the receiving antenna beam. The Receive Terminal automatically acquires the Central Terminal location either by pointing its antenna at the Central Terminal coordinates (latitude, longitude, altitude), or by using known antenna tracking schemes that systematically and automatically position the Receive Terminal antenna to obtain the highest received signal strength, preferably without the need to first demodulate the received signal. When an antenna open-loop pointing method is used (i.e., one that directs the antenna to a point in space without evaluating the signal strength at that point in space), the received signal is rapidly demodulated in order to extract antenna pointing information. In this context the antenna pointing information includes information that is used to point the antenna of the Receive Terminal towards the coordinates of the Central Terminal, i.e., towards the location of the Central Terminal.
The Central Terminal sets the data rate at some Rate Ri(i=0,1, . . . n−1) without the Receive Terminal knowing a priori which rate is being transmitted. One method for receiving and properly demodulating the data is for the Receive Terminal's demodulator and demultiplexer to sequentially attempt to demodulate and demultiplex the received data at each Ri rate. In this case the PN code structure is the same for all data rates, while the rate of the underlying data changes. All rate combinations may be sequentially searched after PN detection/correlation to determine valid data synchronization in the demultiplexer. An unsuccessful attempt to demultiplex the data at a given assumed data rate results in the data rate being changed, and another rate search being initiated. This process continues until the demultiplexing of data is successful.
The foregoing technique has several disadvantages associated with its use: 1) the sequential search (including demultiplexing of data) takes a relatively long time, and 2) the sequential search complicates the spatial search problem for a Central Terminal that may move out of the Receive Terminal's main beam during the sequential search.
It can thus be appreciated that a need exists to provide an antenna spatial search and a data rate discovery method and system where the data rate discovery operation does not impede or slow down the antenna spatial search operation.